Abstract: The mechanisms of brain radiation injury mainly include three hypotheses: vascular injury, glial cells damage and immune response. Most scholars' studies have recently supported the former two ones. Vascular in-jury plays a major role in the effect of delayed radiation injury. Focal brain injury and diffuse white matter injury can be definitely diagnosed by CT and MRI. T₂-weighted imaging (T₂WI) in MRI shows high sensitivity in water contents, and is not affected by the beamhardening arifacts from the cranial base. Compared with CT, the sensitivity of MR for detecting white matter lesions is two to threefold higher. When lesions occrs at the site of an irradiated cerebral tumor, tumor recurrence and focal cerebral necrosis cannot be differentiated by CT or MR, PET and MRS now present a certain advantage of differential diagnosis. Tumoror presents high metabolism and necrosis demonstrates low metabolism by utilizing PET scanning, however PET's sensitivity and specificity are far from satisfactory. The amount or ratio of metabolic products in the region of interest measured by MRS contributes to the deferential diagnosis. In addition, PET functmnal imaging and MRS can also predict the early asymptomatic reversible radiation injury so as to allow the early therapy of steroids andpossibly other drugs, prior to the development of irreversible changes.